Dyno Basics

 Pump Action : 

Since the mechanics of pump operation normally cause the majority of surface load changes during each pump stroke and the pump is the most common source of mechanical failure in rod pumped systems, is important to understand the basic pump action. In fact, it is impossible to make sense of even the the most basic dynamometer trace without some understanding of how a rod pump works. Simply put, the rod pump action must be understood before informative dynamometer analysis can be made.

A brief summary of the basic pump action follows :

     
 
 The above three graphics show the traveling valve (top blue ball) and the standing valve (bottom red ball). When the polished rod moves upward from its lowest position, the TV ball will seat from its own weight. The upward movement of the plunger creates a low pressure in p2 as the pump chamber volume is increased (upstroke).

When p2<pt the standing valve opens and fluid migrates from pt to p2. During this period (upstroke) p2<p1 which causes the load to increase at the top of the rod, where the force is measured by the load sensor. The pressure differential across the plunger remains until the top of the stroke is reached and the direction of rod movement changes.

As the plunger travels downward p2 increases until p2>p1, at which point the tv opens and fluid is displaced from p2 to p1. As p2 increases, the pressure differential across the plunger and the load at the surface (as measured by the dynamometer apparatus) decreases. It should be noted that if the pump barrel is not completely full the pressurization process will not occur until a relatively non-compressible fluid is encountered by the plunger. The location of this encounter is mainly dependent on the pump displacement, fluid inflow into the well bore, and pump slippage, and can be clearly seen from the "dyno trace", discussed below.

The action of the pump thus causes pressure p2 to increase and decrease during each pump cycle which in turn results in the familiar rectangular shape of the load vs. position plot (dynamometer) in wells with full pumps and low pumping speeds.

Dyno Trace:  A plot of polished rod load (y-axis) vs. polished rod position (x-axis). The load signal usually comes from a load sensor either mounted on the rod itself or between the carrier bar and rod clamp. The position signal usually comes from a spring wound potentiometer attached to the polished rod. These signals are typically digitized and stored by a computer as coordinate pairs at the rate of 20 to 30 pairs per second.

The dyno trace consists of all forces applied to the rod including fluid load on the pump plunger, casing and tubing pressures, stuffing box friction, rod harmonics, acceleration and so on. The normal trace refers to a complete pumping cycle obtained when the well is pumping in a stabilized condition whereas the down card refers to a cycle measured after the pumping has been stopped for a brief downtime.

The perfect card shape as shown above represents what would be seen if one were to test a well pumping at a very slow speed with an inelastic rod string and complete pump fillage. While this condition never exists in the real world, there are cases when slow speed / shallow wells produce close approximations to the perfect trace. The remaining traces show how other common real world forces affect the dyno card. 

It is important to note that the load is measured at the top rod. Any force acting on the rod string below the measuring point will be sensed by the dynamometer. Rod weight, stuffing box friction, fluid acceleration, rod harmonics and pressure differential across the plunger are all forces that contribute to the load measured and included in the "dyno trace".

Dynamometer traces can be created by a variety of testing systems including mechanical dynamometers, pumpoff controllers, electronic x-y plotters, and so on. In the case of the system you are currently using, the sister program of Dynoeze, TET (Totally Electronic Test - also by Anatesco) is used as the front end data gathering system. TET is PC based and is designed to generate the data that is input into dynoeze via floppy disk or e-mail. The data that is generated by TET is a text file that represents a single well test, located in the dynos\storage subdirectory and has a ".dyn" extension.

Load Vs. Time:   Similar to the dyno plot with load still on the y-axis but time on the x-axis. This plot, often referred to as "valve checks" is usually a two part graph with the second part position vs. time, with position on the y-axis. This is done so load can be correlated to both position and time. Load vs. time plots are made by stopping the rod several times on both the upstroke and the downstroke during the testing period and recording the resulting loads and positions on a time scale.

A typical valve check graphic can be seen in the "load vs. time" and the "position vs. time" in a typical Dynoeze test.

"Valve checks" are very useful in analyzing the condition of both the standing valve and the traveling valve. When stopping the rod on the upstroke, a good traveling valve will hold the pressure differential across the plunger and the measured load will remain in the upper portion of the graph. If the load drops during the stop, the TV/Plunger is slipping fluid into P2, causing p2 to increase, pressure differential across the TV to decrease, and the surface load to drop.

Conversely, a bad SV can be detected by stopping the rod on the downstroke equalizing the pressure across the TV, which puts the pressure differential across the SV. If the load remains in the lower portion of the graph, the SV is ok. If the SV slips, p2 decreases and the differential across the plunger increases, increasing the load at the surface.

Hint ... A quick and easy way to read the valve checks: 

1. During upstroke stops : the load should go up & stay up  - otherwise the TV is slipping
 2. During downstroke stops : the load should go down & stay down - otherwise the SV is slipping

Pump Fillage:  The rate of fluid entering the pump barrel compared to the total pump displacement rate. If fluid enters the pump barrel at a rate equal to the pump displacement rate, the pump fillage is 100%.

If a well is displacing 800 bfpd and only 400 bfpd enters the pump, the pump fillage will be 50%. If the plunger begins to slip fluid, however, the barrel will contain the 400 bfpd plus whatever fluid slipped past the plunger, so the fillage rate will increase as the pump wear increases. When the fillage reaches 100%, fluid accumulates in the tubing / casing annulus decreasing production.

Pump fillage is directly related to fluid pound, but is the preferred way to express this aspect of pump performance. When using pound location to assess pump performance, the stroke length should be considered as well as the pound location to protect from making an incorrect judgement, whereas a single fillage figure conveys the same information. Example: a well has a pound at 20" from the top. Should displacement be decreased ? If the unit has a 144" stroke, probably not, but on a 21" stroke, probably yes. Given the pump fillage of 86% or 5% is a more direct and easier way to deal with fluid entry into the pump.

Pump Slippage:   The volume of fluid that slips by the plunger or sv compared to the total pump displacement. The volume of fluid that slips past the TV on the upstroke adds to the fillage figure, but represents fluid that has been lifted during a previous stroke, so therefore needs to be subtracted from displacement calculations for accurate net pump displacement. Dynoeze currently indicates fluid slippage by codes: TV, SV, BV, WB are codes for traveling valve, standing valve, both valves, or worn barrel slippage, respectively.

Fluid over pump (FOP):    If  the fluid flow into the wellbore is greater than the net pump displacement rate, fluid will accumulate in the casing / tubing annulus, and there will be ample fluid available to fill the the pump barrel. If this condition exists, then the pump fillage will be 100% and the fluid level is at some point above the pump intake. Less than 100% pump fillage, conversely, means a well is pumped off, or fluid flow into the well is less than the effective displacement rate, and the liquid level in the casing is at the pump intake.

                                                                                                              
 The following formula can be used to calculate fluid over pump using dynamometer loads :

      FOP = ( tp + ( pd * ( 1 - ( sv / ( pd * ard * 2.18 ) / .128 ) * .433 ) - ( tv - sv ) / a ) - cp ) / .433

tp = tubing pressure pd = pump depth sv = standing valve load  tv = traveling valve load

ard= average rod dia a =  plunger area cp = casing pressure  fop = fluid over pump

Card Shape Analysis:    Many pump problems can be easily resolved by simple dynamometer card shape analysis. Card shape analysis is quick and easy. Just a glance at the dynamometer trace can reveal much about the condition of the pump, fluid level, and many other aspects of the pumping system. The idea behind card shape analysis is to relate the card shape to the forces which caused the shape. Once the forces are understood the determination of pumping system performance is reduced to a matter of simple deduction.